What is Corundum ?
What is Corundum and What are its Basic Qualities?
Corundum is an aluminum oxide that commonly forms hexagonal barrel-shaped prisms that taper at both ends or as thin tabular hexagonal plates. It has a hardness of 9 on the Mohs scale, making it one of the most durable commercial gemstones. It has no dominant cleavage and fractures in a conchoidal manner. A high density of ~4.0 g/cm3 (most silicate minerals are ~2.6 g/cm3) results in corundum occurring in secondary placer deposits and recoverable by panning methods, similar to how you would recover placer gold.
Corundum comes in all colours of the rainbow but is most commonly found as opaque crystals with dull colours. Red corundum is called ruby, blue corundum is called sapphire, and all other colours are called fancy sapphires. Some varieties of corundum will fluoresce under short wave and long wave UV light if there is enough chromium in the crystal structure but little iron, which tends to quench any emitted energy.
What Colours can Corundum Have, How are These Colours Generated, and What Gem Varieties Result?
Pure corundum is colourless and clear if transparent or pale white if opaque. This mineral also has low dispersion so the value of the stones comes not from fire generated (as in diamond), but rather from the intensity of colours seen. The vivid colours of corundum gem varieties, such as ruby and sapphire, arise primarily from elemental substitution in the Al site by transition metal elements. The most common cations to substitute are Fe+2, Fe+3, Ti+4, Cr+3, and V+3.
A continuum of colour saturation exists between pink sapphire and ruby that is correlated with trace amounts of Cr. There is no official cutoff for the amount of Cr needed for ruby, but usually rubies will have up to ~1 wt% of Cr2O3. When Cr substitutes for Al, wide absorption bands are generated in the violet (~450 nm) and green-yellow (~500 nm) ranges, as well as overlap a bit into the blue region. The red region of the electromagnetic spectrum (~650 nm) does not have very much absorption at all and results in all colours but red being blocked by ruby.
But there is another trick up ruby's sleeve that makes its red almost jump out at the observer. When Cr is introduced into corundum it makes the mineral fluorescent under UV light. This means that UV energy from normal light is accepted into the crystal and then re-emitted at a lower energy level - conveniently in the red region, thus amplifying the intensity of red in ruby under daylight conditions. However, if any iron is present it will usually absorb the red fluorescence from UV light. Thus, the finest rubies are those that have little to no iron in their crystal structure.
Blue sapphires are generated primarily from pairs of Fe+2 and Ti+4 substituting into the crystal structure for Al+3. The process of intervalence charge transfer (essentially continual swapping of electrons, bouncing back and forth) occurs between the Fe and Ti and all colours except blue are absorbed. So like ruby, it is the absorption of all other colours from full spectrum light (aka white light) that generates the beautiful blues in sapphires, rather than the "generation" of the blue colour. Very small amounts of these elements (only ~0.01 wt% Fe and Ti) are needed to produce the vivid blues.
Other colours are generated from a combination of these elements, as well as other minor cations and defects in the crystals. Also, a single corundum gemstone can be multi-coloured from different concentrations of metals in different parts of the crystal - this is called zoning.
Some sapphires also show an optical characteristic called asterism, which is most commonly seen as a six or twelve pointed star. These "arms" of the star are generated from oriented inclusions of long and skinny minerals (almost always the mineral rutile, a titanium oxide, TiO2). Specimens found with these inclusions are often cut and polished in a rounded and polished cabochon style to emphasize the nature of this optical effect. Rutile inclusions can occur in both sapphires and rubies, although it is more common in sapphires.
How is Corundum Valued?
Rubies and sapphires are valued primarily for their colour, then for their clarity. Since rubies and sapphires are treasured for their intense colour it is no surprise that this is the primary deciding factor for its value. A nice clean stone is more attractive than one that is heavily included. Origin has a strong impact on the value of stones; size and cut are also important.
Stones originating from conflict zones or undisclosed locations are often undesirable to many consumers. Conversely, corundum sourced from historical locations are, in a sense, analogous to brand name items like Gucci or Armani. When considering size, large stones are rarer. For cut, a poorly faceted stone will not show its best colours and will likely have to be re-cut and therefore lose some carat-weight. So, like diamonds, there are 4+1 Cs to evaluating gem corundum: colour, clarity, cut, carat… plus Country of origin!
The best sapphires are valued according to the purity and intensity of the blue, with the "ideals" showing either refreshing cornflower blue or a velvety royal blue. Most of the highest calibre sapphires come from three different regions of the world. Stones from Kashmir are often the most valuable and exhibit an intense, velvet-like blue. Burmese sapphires are also highly valued for their saturated blue. Burmese stones often show wonderful asterisms. Finally, Sri Lanka and its cornflower blues, not to mention their very large sizes, are also prized.
Rubies with Pigeon's Blood red colouration, a colour described as a pure red with a hint of blue, are highly valued. Pigeon's Blood red rubies typically originate from Burma, but other noteworthy localities of high quality ruby are found in Vietnam, Sri Lanka, and Thailand. The highest quality rubies will show a strong red fluorescence and sometimes contain fine rutile silk that scatters the light across the stone, displaying a full bodied colour. Rubies with the finest optical qualities (colour, clarity) rarely have significant weights and a stone of ~2 carats is considered quite large.
The fancy sapphires (any colour of corundum other than blue or red) are more volatile in value and are driven by the consumer market place. For instance, fancy hot pink sapphires spiked in value over the last ~5-10 years whereas colourless, yellow, green, and orange stones have not received as much attention from consumers. One exception to this are Padparadscha sapphires that have an orange-pink colouration. These stones are known to have prices that approach the levels of fine rubies.
Once examined by a gemologist, rubies and sapphires will be ascribed a rating based on the 4+1 Cs. Ratings for coloured stones are less comprehensive than that for diamonds, and the five usual categories used are Poor, Fair, Good, Very Good, and Exceptional. In most jewellery stores, the top stones will be of "Good" quality. Very Good stones are found in high end stores, and Exceptional stones are found only in exceptional circumstances. The following tables are estimates of prices per carat for "Good" rubies and sapphires from "non-prime sources" based on size (prices accurate as of ~2007). Note how the value per carat increases with increasing size.